Since there was a lot of interest in this thread, Mike added a page to the docs with an ECharts example: https://observablehq.com/framework/lib/echarts
There are two pieces of that example code specific to Framework: the html`` tagged template literal creates a DOM element (see https://github.com/observablehq/htl, also usable outside Framework), and the display function inserts it into the document above the code block (see https://observablehq.com/framework/javascript/display). Note that, whereas Observable Plot takes an options object and returns a DOM element, ECharts instead takes a DOM element and mutates it — but in general they should be equally easy to use in Framework.
Like Plot (and Vega-Lite, another great option), ECharts is also now one of Framework’s built-in “recommended libraries” (see https://observablehq.com/framework/javascript/imports#implic...), meaning that if you reference `echarts` Framework will lazy-load it for you. Adding that was a two-line diff: https://github.com/observablehq/framework/pull/811/files#dif.... But I wanna emphasize that Framework doesn’t have to explicitly “support” a given library for you to use it. “Supporting” in this case just means the convenience of saving you a one-line import statement. But don’t wait for our blessing!! Use whatever.